Conclusion

- Evolution and medicine: the long reach of "Dr. Darwin"

It is clear that evolutionary biology has an enormous potential to
enrich our understanding of biomedical phenomena. It is also clear that
the study of biomedical phenomena can greatly enrich our understanding
of evolutionary processes. These observations should be of relevance to
biological and biomedical investigators and educators. Moreover, the
examples drawn from immunology and oncology show that the human body
itself is a laboratory for fast evolution. This fact has significant
philosophical implications for the philosophy of science, especially as
it relates to the nature of explanations in the biological sciences.

Much of this review has been devoted to ways in which evolutionary
biology can enrich our understanding of biomedical phenomena. However,
the study of biomedical phenomena shows the need to rethink some
aspects of evolutionary biology. Traditional Darwinists draw a sharp
distinction between mechanistic explanations on the one hand, and
evolutionary explanations on the other. Thus, in dealing with the
question, "What is biology?" the great evolutionary biologist Ernst
Mayr observed:

When we try to answer this question, we find
that biology actually consists of two rather different fields,
mechanistic (functional) biology and historical biology. Functional
biology deals with the physiology of all activities of living
organisms, particularly with all cellular processes, including those of
the genome. These functional processes ultimately can be explained
purely mechanistically by chemistry and physics [[39]:24].

But the story does not end so simply; Mayr continues:

The other branch of biology is historical biology.
A knowledge of history is not needed for the explanation of a purely
functional process. However, it is indispensable for the explanation of
all aspects of the living world that involve the dimension of
historical time – in other words, as we now know, all aspects dealing
with evolution. This field is evolutionary biology [[39]:24].

Mayr observes that the most frequently asked question is mechanistic
(functional) biology is "how?" whereas the most frequently asked
question in evolutionary biology is "why?" He adds, "To truly
appreciate the nature of biology one must know the remarkable
difference between these two branches of biology" [39].

Focusing their attention on the contours of the new science of
Darwinian medicine, traditional Darwinists Nesse and Williams
distinguish between two types of causes that are medically relevant
(and thus require two different types of causal explanation):

Consider heart attacks. Eating fatty foods and
having genes that predispose to atherosclerosis are major causes of
heart attacks. These are what biologists call proximate ("near")
causes. We are more interested here in the evolutionary causes that
reach further back to why we are designed the way we are. In studying
heart attacks, the evolutionist wants to know why natural selection
hasn't eliminated the genes that promote fat craving and cholesterol
deposition. Proximate explanations address how the body works and why
some people get a disease and others don't. Evolutionary explanations
show why humans, in general, are susceptible to some diseases and not
to others [[12]:6].

The distinction is between mechanistic explanations that answer
"how" questions, and evolutionary explanations that answer "why"
questions. Evolutionary explanations are typically viewed as long-term,
historical explanations (one might have to consider the entire course
of human evolution, for example), whereas mechanistic explanations are
immediate – and for many purposes, essentially ahistorical.

It is true that looking at medical phenomena from the standpoint of
traditional Darwinism typically means taking a historical perspective –
and as we have seen above, it certainly has a legitimate role to enrich
our understanding of biomedical phenomena. It is also true that
traditional Darwinists recognize that rapid evolution is possible for
organisms with short generation times, such as viruses and bacteria –
organisms where the relevant history may concern events occurring over
the course of a few months. But we now see that traditional Darwinism
is only a part of the Darwinian medical story. Consideration also needs
to be given to the role of Darwinian explanations of biomedical
phenomena occurring in the life-cycles of animals – including humans.

The examples we have presented concerning the role of Darwinian
explanations in the realms of immunology and oncology show that it is
not easy to draw a sharp distinction between mechanistic explanations
and evolutionary explanations. For phenomena in the domain of humoral
immunity and oncology, important aspects of the mechanistic explanation
involve a rapid evolutionary explanation. In this way the study of
biomedical phenomena shows the need for a critical reassessment of
generalizations about the nature of biological explanation that have
been forthcoming from traditional Darwinists.